Against this backdrop of continued fighting in Aleppo and other provinces, Stephen O'Brien, the UN humanitarian chief, called on Monday for immediate "unconditional and unhindered" humanitarian access to an estimated 5.5 million Syrians in besieged or difficult-to-reach areas.

More than 280,000 people have been killed in Syria and more than half its population has been displaced since the conflict began.

Boys salvage goods from the wreckage left behind after an air strike hit the rebel-held town of Atareb in Aleppo province. [Ammar Abdullah/Reuters]

People inspect the damage at a site hit by air strikes in the rebel-held town of Atareb in the countryside of Aleppo. [Ammar Abdullah/Reuters]

Civil defence members transport a casualty after an air strike hit the rebel held area of Douma, near the capital. [Bassam Khabieh/Reuters]

Nearly 600,000 people live under siege in Syria, most of them encircled by President Bashar al-Assad's forces, whose approval the United Nations says is needed to deliver aid by air. [Bassam Khabieh/Reuters]

The UN's top aid official has called for a humanitarian truce in the city of Aleppo, where nearly 400,000 civilians are now under government siege. [Ammar Abdullah/Reuters]

A man takes pictures with his mobile phone at a site hit by an air strike. [Bassam Khabieh/Reuters]

Civil defence members and civilians search for survivors after an air strike in rebel-held Douma. [Bassam Khabieh/Reuters]

The Syrian uprising started with largely unarmed protests against President Bashar al-Assad in March 2011, but quickly turned into a full-blown civil war that has since continued unabated. [Bassam Khabieh/Reuters]